The treatment of used rubber and plastics such as vehicle tires, car fluff (plastic and rubber residues from shredded automobiles), and RDF (Refuse Derived Fuel) is a major environmental problem.
Vacuum pyrolysis of rubber and plastics cuttings is state of the art. For example, U.S. Pat. No. 4,740,270 describes a process for the vacuum pyrolysis of scrap tires in which tire cuttings are moved in a multi-tray reactor with conventional transportation system of the tire cuttings from an upper to a lower tray for subjecting them to pyrolysis. The tire cuttings are shifted from the top tray to the bottom tray along a temperature gradient starting at 200.degree. C. and peaking at 500.degree. C., the tire cuttings being thus heated up continuously while moving down. The process aims a high oil yield against non condensable gas, and to achieve that result, the document recommends a subatmospheric pressure of less than 35 mm Hg. It is known that rubber undergoes a softening stage before pyrolysis sets on from 200.degree. C. onwards. Softened tire cuttings are sticky and prone to fix to any available surface, thereby choking the equipment. In the multi-tray reactor described, the tire cuttings will stick in one stand, most likely always the same tray, as long as the speed of transportation, the temperature gradient, and the heat transfer coefficient are maintained constant. The rubber cuttings may therefore choke a tray, whereas the transport of the cuttings is impaired and the pyrolysis gas is trapped. An overpressure is created in the reactor which may explode. Furthermore, to maintain the aforesaid subatmospheric pressure of 35 mm Hg, the reaction rate must be carefully controlled and kept low, which implies a long residence time of the materials in the reactor and therefore the reactor must be large. Still further, large reactors are expensive, and the lower the pressure in the reactor the more powerful and expensive the vacuum pump. All these adverse features make the equipment expensive and hazardous.
It is an object of this invention to overcome these drawbacks by means of a process for the treatment of used rubber and plastics by vacuum pyrolysis which is safe, efficient, and economical.